Composite materials are contemplated for use in reducing the structural weight and increasing the strength and fatigue life of contemporary and future aerospace vehicles. It is anticipated that research by government and industry to improve these physical property characteristics of composites will continue for many years. One of the problems inherent in composite structures has been the fatigue life of epoxy-matrix composites and efforts to improve or increase this and other physical properties of composites are continuing.
Fracture and notch-strength tests of graphite-epoxy composites have shown that unidirectional layups generally exhibit longitudinal cracking before failure, whereas multidirectional layups fail transversely with little longitudinal cracking. By a simple qualitative analysis it has been shown that the higher matrix shear stresses in unidirectional materials cause the longitudinal cracking and that this cracking is responsible for the increased toughness of unidirectional components.
Although graphite-epoxy composites have a high strength-to-density ratio, they lack ductility and are more notch-sensitive and more damage-sensitive than many common metallic materials. Notch-strength, fracture toughness, and impact resistance are of concern in primary structures, and several methods have been proposed previously to improve these properties. For example, hybrid composites use combinations of various fibers, such as fiberglass and graphite, or Kevlar and graphite to increase the impact resistance and notch strength. In some prior art hybrids the lower modulus fibers (glass or Kevlar) and the graphite fibers are confined to separate laminae, while in others they are mixed within each laminae. This hybridization also increases the fracture toughness of the composite. The softening-strip method uses lower modulus fibers in specific areas such as fastener rows to increase notch-strength and toughness. In all of these prior art systems, the strength-to-density ratio is lower than for graphite-epoxy alone.
It is an object of the present invention to improve the notch-strength and toughness of a composite structure without resorting to lower modulus fibers.
It is a further object of the present invention to provide a new and improved composite structure.
It is another object of the present invention to provide a novel process for making composites that have improved interlaminar strength property characteristics.
An additional object of the present invention is an improved composite structure that has improved physical property characteristics of toughness, notch-strength and impact strength.
According to one aspect of the present invention the foregoing and other objects are attained by placing thin perforated polyester films between some or all of the laminae in a composite layup and curing the composite layup to cause resin flow through the perforations for bonding of the individual laminae.